1. Field of the Invention
The present invention relates to an image processing apparatus and a method of controlling the same and, more particularly, to an image processing apparatus for image reduction processing and a method of controlling the same.
2. Description of the Related Art
Equipping an image capturing apparatus such as a digital camera with an image sensor such as a CCD image sensor or a CMOS image sensor having more than 10,000,000 pixels is no longer a rare case. The growth in the number of pixels leads to growth of the image data amount. For this reason, processing that does not need to use all pixels obtained by image capturing is preferably executed after reducing the original image in terms not only shortening the processing time of course but also saving power and hardware resources such as memory.
A general single board color image sensor is provided with color filters each having a specific color for a pixel. Each pixel receives transmitted light from a color filter of one color. As the color arrangement pattern of the color filters, a Bayer arrangement having a 2×2 pixel pattern is widely known in which green (Gr and Gb) pixels are arranged for a pair of diagonal pixels, whereas red (R) and blue (B) pixels are arranged for the remaining two pixels, respectively. In the image sensor using the color filters, only single-color signal information is obtained in each pixel. To obtain a color image, it is necessary to perform processing (also referred to as a synchronization process) of interpolating signal information of a plurality of colors (R, Gr, Gb, and B) in each pixel using the information of neighbor pixels of different colors.
Hence, the two-dimensional spatial-frequency bandwidth capable of reproducing each color is affected by the arrangement of the color filters. Assume that, for example, color filters having the Bayer arrangement are used. Defining the size in the vertical and horizontal directions as 1, the size of the frequency bandwidth capable of reproducing green is 1/√2 in the 45° oblique direction. Defining the size in the vertical and horizontal directions as 1, the size of the frequency bandwidth capable of reproducing red and blue is √2 in the 45° oblique direction. The size of the frequency bandwidth capable of reproducing green is equal to that of red and blue in the 45° oblique direction but twice larger in the vertical and horizontal directions.
Image reduction is implemented by decreasing the number of pixels using thinning out the pixels or interpolation/composition to form one pixel from a plurality of pixels. To suppress aliasing noise generated by high-frequency components upon thinning out or interpolating pixels, the high spatial-frequency components of the original image are removed using a bandpass filter (low-pass filter) before processing.
Japanese Patent No. 4096626 discloses an arrangement for obtaining a high-quality reduced RAW image by controlling the band characteristic of the low-pass filter to be applied to each color pixel when obtaining a reduced image (reduced RAW image) from an image (to be referred to as a RAW image) before the synchronization process in which each pixel has single-color signal information. More specifically, there is disclosed an arrangement using a low-pass filter whose band characteristic corresponds to reduction ratio ×sampling frequency fs/2 and is similar to the frequency bandwidth capable of reproducing each color. For a RAW image obtained by an image sensor using, for example, color filters having the Bayer arrangement, a low-pass filter that reduces the bandwidth in the 45° oblique direction is applied to the G component, and a low-pass filter that does not reduce the bandwidth in the 45° oblique direction is applied to the R and B components. This makes it possible to obtain a high-quality reduced RAW image capable of suppressing false color generation in the color image after applying image processing such as a synchronization process.
In the technique described in Japanese Patent No. 4096626, however, the RAW image is directly input to the low-pass filter. For this reason, when the RAW image includes many horizontal pixels, the line memory capacity necessary for implementing the low-pass filter also increases. To reduce an image in the vertical direction, the line memories need to be arranged in the vertical direction. Hence, if the capacity per line increases, the total capacity of line memories necessary for two-dimensional reduction increases more and more.